This invention relates to a process for recovering ammonium sulphate from an aqueous solution contaminated with organic substances formed in the synthesis of organic compounds. The invention relates in particular to the recovery of ammonium sulphate formed in the preparation of acrylonitrile from propylene, of methyl methacrylate from acetone cyanohydrin, and of methyl acrylate from acrylonitrile.
The presence of contaminants consisting of primarily tarry and/or sulphur-containing organic substances, which are generally of little value and are considered as waste matter, causes difficulties and increased costs in working-up ammonium sulphate solutions which are formed in the preparation of organic compounds. Avoiding additional costs by simply discharging these solutions as waste water is generally not feasible in view of the resulting water pollution, and the recovery of pure ammonium sulphate by crystallization is impeded by the presence of the organic substances; impure products will be obtained and the costs of the purification will usually exceed the value of the ammonium sulphate obtained.
It has been previously proposed to recover ammonium sulphate from an aqueous solution containing solid organic components by a process in which water is evaporated from the said solution and an organic phase of the contaminants is formed and removed, after which ammonium sulphate is recovered from the remaining aqueous solution by crystallization. This process for purifying ammonium sulphate solutions can only be applied if the required phase separation between the ammonium sulphate solution and the contaminants takes place as a result of the evaporation of water from the solution. An example of the application of this prior process is the working-up of the ammonium sulphate solution left over in the preparation of caprolactam.
The above prior art process is not, however, applicable to other ammonium sulphate solutions, such as the solution formed in the preparation of acrylonitrile by the oxidation of gaseous propylene in the presence of ammonia. The oxidation off gas, which contains an excess of ammonia, is treated with sulphuric acid, in which treatment a waste flow is formed of ammonium sulphate, which is contaminated with organic components, in particularly with tarry impurities. Similar contaminants are contained in the ammonium sulphate solutions formed in the preparation of methyl methacrylate from acetone cyanohydrin, methanol and sulphuric acid, or of methyl acrylate from acrylonitrile, methanol and sulphuric acid. Treatment of these solutions with common solvents, such as acetone, benzene, ethyl alcohol, carbon tetrachloride, dichloroethane, cyclohexane, cyclohexanone, methyl isobutyl ketone, etc., is not effective because such solvents leave the organic contaminants wholly or partly undissolved.